Geology and Topography of Western New York. 87 
evidently been exposed. The deep valleys, which penetrate this 
formation in a southerly direction from the great limestone terrace; 
the dividing ridges, also, which have their northern terminations 
on the same terrace, becoming more rugged and mountainous as 
they approach the Pennsylvania state line, with their sides deeply 
furrowed by precipitous gullies and ravines, are sufficient proofs 
that other ‘causes of denudation than the insignificant streams 
which traverse these valleys, have been in operation. 
One peculiar feature, which adds greatly to the picturesque 
scenery of Western New York, arises from the fact that many of 
these valleys have. been excavated to a level below the general 
escarpment of the limestone terrace,* which consequently forms 
a barrier at their mouths, and gives rise to most of those beautiful 
_ sheets of water so justly admired by the lovers of fine scenery. 
This feature will again be alluded to further on. 
The aggregate thickness of the rock strata, from Lake Ontario 
to the northern outcrop of the coal in Pennsylvania, is estimated 
by Mr. James Hall at six thousand and fifty one feet.t| How far 
they extended to the north, and whether the primitive regions on 
either or both sides of the St. Lawrence, were originally overlaid 
by them, are questions difficult to solve; and which require se 
minute and careful examination of the geologist. There 
some circumstances, however, which seem to favor this fonts 
sion. It is stated by Dr. Bigsby,t when speaking of the hori- 
zontal limestone of the Canadas, that “this limestone forms a 
horizontal~girdle around the trap mountain of Montreal, from 
which, as from a centre, large veins or dykes radiate into the ad- 
joining limestone to the distance of two miles in some cases to 
my own knowledge, and even to La Chine, according to informa- 
tion received from M. Burnett, chief engineer to the La Chine 
es The limestone in its upper strata, is brown and crystal- 
‘line, but black, compact, and slaty below. It contains in immense 
quantities the organic remains peculiar to the mountain limestone 
of oe api and Ireland.” It is also stated’ — Prof. ibis S$ 
.* Since writing the above, my attention has been called to the fact that Mr. 
David Thomas commu niet s phenomenon to Prof. Eaton in 1830: See Amer- 
ican Journal, Vol. xviii. p.: 
t New York “Goolodical Report; 1838. -See Atlas. 
» American Journal 
te > Vole Wa. pe vd. 
»~ § New York Geological Report, 1838, p. 255. 
